1. Field of the Invention
This invention relates to a semiconductor light emitting element that is structured such that an ohmic contact joint part and a reflection metal film are sandwiched between a group III-V compound semiconductor layer including a light emitting layer and a support substrate. In particular, this invention relates to a semiconductor light emitting element with improved light extraction efficiency.
2. Description of the Related Art
Recently, for light emitting diodes (LEDs) as a semiconductor light emitting element, GaN or AlGaInP-based high quality crystals can be grown by metal-organic vapor phase epitaxy (MOVPE), so as to produce high brightness blue, green, orange, yellow and red LEDs. Along with the development of the high brightness LEDs, its use has been expanded to a brake lamp of automobile, a backlight of liquid crystal display and the like, and its demand has been increased year by year.
At present, since high quality crystals can be grown by MOVPE, the internal efficiency of the light emitting element comes close to the theoretical value or the limit value. However, the light extraction efficiency of the light emitting element is still low, so that it is more important to improve the light extraction efficiency. For example, a high brightness red LED is formed of an AlGaInP based material, and the light emitting part thereof forms a double-hetero structure that has an n-type AlGaInP layer and a p-type AlGaInP layer of an AlGaInP based material with a composition lattice-matching on a conductive GaAs substrate, and a light emitting layer (or an active layer) of AlGaInP or GaInP and sandwiched between the n-type AlGaInP layer and the p-type AlGaInP. However, since the bandgap of the GaAs substrate is narrower than that of the light emitting layer, most of light emitted from the light emitting layer is absorbed by the GaAs substrate such that the light extraction efficiency is significantly reduced.
For reducing the light absorption by the GaAs substrate, the following methods are used. A method is known that the light absorption by the GaAs substrate can be reduced by forming a multilayer reflection film structure with semiconductor layers different in refractive index between a light emitting layer to enhance the light extraction efficiency. However, in this method, only light with a restricted incident angle to the multilayer reflection film structure can be reflected.
Further, a method is proposed that a double-hetero structure of an AlGaInP based material is bonded to a Si support substrate with a thermal conductivity higher than the GaAs substrate via a reflection metal film with high reflectivity, and the GaAs substrate used for growth of semiconductor is then removed. This technique is disclosed in, for example, JP-2005-175462 A1. By the reflection metal film used for this method, high reflectivity can be brought regardless of any incident angle of light to the reflection metal film, so that high brightness LED can be obtained.
In the above LED structure that the reflection metal film is sandwiched between the light emitting layer and the Si support substrate, when a high reflectivity metal such as Al, Au, Ag is used for the reflection metal film, it is impossible to have electrically ohmic contact with the compound semiconductor. Therefore, a part of the reflection metal film is replaced by ohmic contact joint part. Thus, electron or hole injected into the LED element moves through a front surface electrode formed on the light emitting part and the ohmic contact joint part to the Si support substrate. At the time, light is emitted from the active layer located between the front surface electrode and the ohmic contact joint part. The emitted light is extracted through a surface (i.e., a light extraction surface) of the semiconductor layer on the light emitting part outside the light emitting element.
In the LED structure that the ohmic contact joint part and the reflection metal film is sandwiched between the light emitting layer and the Si support substrate, a part of light emitted is multiply reflected between the light extraction surface and the reflection metal film. If the reflected light reaches the ohmic contact joint part or the front surface electrode, the ohmic contact joint part and the front surface electrode highly absorb the emitted light, so that the light cannot be extracted outside. Particularly, the front surface electrode has an area larger than the ohmic contact joint part such that it can be a big absorption factor relative to the whole light emitting element. Thus, the area of the front surface electrode needs to be reduced. However, the front surface electrode having a simple circular shape etc. may cause a further deterioration in current spreading property if the area of the front surface electrode is reduced.
In order to uniformly spread current in the light emitting element, various shapes of the front surface electrode are devised, for example, linear electrodes being extended from the center of a circular portion (e.g., as shown in FIG. 10), so as to attain not only the enhancement of the light extraction efficiency but also the reduction of forward voltage.